Tunable ultrafast light sources are important for various spectroscopic and microscopic applications, such as the pump-probe spectroscopy, fluorescence lifetime imaging microscopy (FLIM), and multiphoton microscopy/spectroscopy. Many previous applications were based on a Kerr-lens mode-locked Ti:sapphire laser. However, the tuning range of a Ti:sapphire laser is limited to around 700 nm-1000 nm because of the laser gain bandwidth of the Ti:sapphire crystal. To extend the wavelength range of an ultrafast laser, supercontinuum generation (SCG) or optical parametric amplification (OPA) has been adopted.
The so called supercontinuum is a light source with an extra-wide bandwidth, and SCG is capable of extending input laser wavelengths into both shorter and longer wavelength directions about hundreds of nm. Thus, the purpose of extending the wavelength range is realized. However, as a result of extending of the wavelength, the pulse energy spreading in each wavelength range will be reduced in a corresponding way. Nevertheless, inasmuch as SCG redistributes the pump power into a wide wavelength range, the power density of SCG is typically less than 1 mW/nm. In addition, the strong chromatic dispersion in the fibers significantly lengthens the pulses, and subsequently degrades the applicability of the SCG because of the reduced peak power and poor temporal characteristics.
Both energy conservation and phase-matching condition have to be fulfilled when a shorter-wavelength pump wave is down-converted into a signal and an idler wave simultaneously. If a seed having the same wavelength with the signal or the idler is provided, the probability of conversion would be raised higher due to being excited, and thus the density of the output signal and idler were amplified. Since the wavelengths of the output signal and idler are different from that of the pump, thus the wavelengths can be extended, and the wavelengths are adjusted by changing the conditions of phase-matching condition. Generally speaking, it is hard to find a seed which is continuously tunable from ultraviolet to near-infrared, so the systems designed by the principle of optical parametric amplification are most likely complex and huge. In the conventional implementation, the conversion efficiency of such system is still low, and thus a high-energy, low-repetition rate pump source or an oscillator cavity is required to improve the conversion efficiency, in which the signal and the idler vibrate back and forth due to resonance to accumulate profits and reduce the pump threshold in the optical parametric conversion process.
In the above-mentioned methods, regardless of raising laser energy, lowering the repetition rate or setting the oscillator cavity, not only the high cost and complex system are problems, but also for some industries, the low repetition rate is insufficient to handle the requirements in use. However if not so, the conversion efficiency can not be raised effectively. It is in a dilemma for a person ordinarily skilled in the art.
In order to overcome the drawbacks in the prior art, a wavelength tunable single-pass optical parametric amplifier is provided. The particular design in the present invention not only solves the problems described above, but also is easy to be implemented. Thus, the present invention has the utility for the industry.